NEED HELP - weird tail light/brake light/blinker light/hazzard light problem

I have a '99 Tahoe with a bizarre blinker/brake/tail light issue.

From the beginning:
Fast flash on the right blinker: replaced all bulbs on right rear side - no results. Replaced light assembly on right side & blinker bulb (only) on left side.

Worked perfectly for 3 blocks & problem began again only worse. Turn on the right blinker and the left blinker flashed. Turn on the left blinker and the right side flashed as well as the left side of the license plate. Turn on the head lights and all rear functioning lights become alarmingly dim.

hit the breaks and the turn signal and rear hazard lights come on.

I had recently used my trailer light connector. Years ago it was problematic so I removed it, taped off the ends & double checked grounds.

Next: Replaced the flasher relay. All rear lights appeared to work as designed except fading ~badly~ when head lights were engaged. Tryied again and all worked perfectly and continued to work perfectly for about a month.

Then, while driving at night, I braked & signaled...got a fast flash again. Only this time - I have no tail lights, brake lights or reverse lights but blinkers (dimly) work.

Replaced flasher relay again...I now have brake lights but the flashers are wacked out again and if I turn on the headlights everything goes haywire again.

replaced the mulit function switch.  It lasted about 30 days and all problems started again, PLUS the horn stopped working.

tested all wiring to and from lighting assemblies- all good.

removed entire trailer light assempbly and all power up to that point is correct.

Have never blown a fuse

replaced the head light switch (with a used one) and all problems were the same except when pressing the brake the reverse lights came on.

Put the old one back in and back to all the other problems (sans reverse lights).

In between getting the multi function switch replaced and the problem reoccuring, battery had to be replaced and at the same time new tips were put on the cables.  

Me, and two mechanics are at a complete loss.

Any ideas?